Silicon carbide (SiC) is expected to be a material for a next generation power semiconductor device. In comparison with Si, SiC has excellent physical properties such as threefold band gap, about tenfold breakdown field strength, and about threefold thermal conductivity. Utilizing these characteristics allows an ultralow loss and high temperature operable power semiconductor device to be realized. As one of a various high breakdown voltage semiconductor device, an MOSFET having an upper/lower electrode structure having an n+-type source region, a p-type well region, an n-type drift region on a surface side of a SiC substrate and an n+-type drain region on a back side is illustrated. A mobility of an inversion layer electron of the MOSFET (channel mobility) is lower than a value expected from a material limit of SiC. In order to improve an element performance, namely to reduce an element resistance, it becomes necessary to improve the channel mobility. Until now, the improvement of mobility has been realized in a stepwise fashion by changing a forming condition of a gate insulating film. Further improvement is required.